The present invention relates, in general, to polymers that absorb aqueous liquids (such as water, blood, and urine) and especially, relates to superabsorbent polymers, which are those absorbent polymers that are capable of absorbing over 10 times their weight in water. More particularly, the present invention relates to pre-superabsorbent polymers that will, upon being subjected to radiation, such as heating, cross-link to form superabsorbent polymers.
General background on the manufacture of superabsorbent polymers can be seen in the journal article, xe2x80x9cKeeping Dry with Superabsorbent Polymersxe2x80x9d, Chemtech, (September 1994) by Buchholz. This article contains an excellent discussion of the conventional methods for making superabsorbent polymers, certain of which have sulfonate functional groups and certain of which have carboxylic acid functional groups. Also mentioned are various uses for superabsorbent polymers, such as in a disposable diaper, in a sealing composite between concrete blocks that make up the wall of underwater tunnels, and in tapes for water blocking in fiber optic cables and power transmission cables.
More general background with respect to various superabsorbent polymers and their methods of manufacture can be seen in U.S. Pat. No. 5,229,466 (issued Jul. 20, 1993) to Brehm and Mertens; U.S. Pat. No. 5,408,019 (issued Apr. 18, 1995) to Mertens, Dahmen and Brehm; and U.S. Pat. No. 5,610,220 (issued Mar. 11, 1997) to Klimmek and Brehm, all of which patents are assigned to Chemische Fabrik Stockhausen GmbH.
Another good background discussion of the methods for making superabsorbent polymers can be seen in U.S. Pat. No. 5,409,771 (issued Apr. 25, 1995) to Dahmen and Mertens, assignors to Chemische Fabrik Stockhausen GmbH. More specifically, this patent mentions that commercially available superabsorbent polymers are generally X-linked polyacrylic acids or X-linked starch-acrylic-acid-graft-polymers, the carboxyl groups of which are partially neutralized with sodium hydroxide or caustic potash. Also mentioned is that the superabsorbent polymers are made by two methods. One method is the solvent polymerization method and the other method is the inverse suspension or emulsion polymerization method.
In the solvent polymerization method, an aqueous solution of partially neutralized acrylic acid, for instance, and a multi-functional network X-linking agent is converted to a gel by radical polymerization, typically followed by a heat treatment. The resultant is dried, ground and screened to the desired particulate size.
In the inverse suspension or emulsion polymerization method, an aqueous solution of partially neutralized acrylic acid, for instance, is dispersed in a hydrophobic organic solvent by employing colloids or emulsifiers. Then, the polymerization is started by radical initiators. Water is azeotropically removed from the reaction mixture after completion of the polymerization, typically followed by a heat treatment. The resultant product is then filtered and dried. Network X-linking is typically achieved by dissolving a polyfunctional X-linking agent in the monomer solution.
More specifically with regard to use of heat to effect X-linking, EP Patent Application Publication No. 0 397 410 A2 (published Nov. 14, 1990) to Allen, assignor to Allied Colloids Limited, describes a water soluble, substantially linear, polymer made by co-polymerization of a water soluble blend of monoethylenically unsaturated monomers comprising carboxylic acid monomers such as acrylic acid and a hydroxylic monomer of the formula CHR1xe2x95x90CR2xe2x80x94Yxe2x80x94Maxe2x80x94OH, where R1 is hydrogen or carboxy; R2 is hydrogen, carboxy, or methyl; Y is oxygen, CH2O, or COO; M is alkyleneoxy; and a is at least 5. After the polymer is shaped by extrusion or other shaping of an aqueous solution of the polymer, the polymer is X-linked, such as by heating above 150xc2x0 C., typically at 220xc2x0 C., to form X-linkages between the carboxyl and hydroxyl groups. The shaped resultant is described as being useful for diapers, catamenial appliances (i.e., sanitary napkins), incontinence pads, and bandages. EP Patent Application Publication No.0 397 410 A2 has priority to GB 8910788, which is one of several GB applications to which U.S. Pat. No. 5,147,956 (issued Sep. 15, 1992) and U.S. Pat. No. 5,280,079 (issued Jan. 18, 1994), both assigned to Allied Colloids Limited, have priority.
Moreover, EP Patent Application Publication No. 0 397 410 A2 states that achieved are higher and more reproducible absorption characteristics than in EP Patent Application Publication No.0 268 498 A2 (published May 5, 1988) to Allen, Farrar, and Flecher, assignors to Allied Colloids Limited. EP No. 0 268 498 A2 is a counterpart of U.S. Pat. No. 4,962,172 (issued Oct. 9, 1990), assigned to Allied Colloids Limited. Each of U.S. Pat. Nos. 5,147,956 and 5,280,079 is a Continuation-in-Part leading back to the U.S. application that matured into U.S. Pat. No. 4,962,172.
Additionally, U.S. Pat. No. 4,057,521 (issued Nov. 8, 1977) to Gross, assignor to The Dow Chemical Company, shows water swellable absorbent articles, made from copolymers having a copolymerized crosslinker, together with methods for their preparation, and a composition containing a copolymerized crosslinker useful to make said articles. The articles are crosslinked by heating and/or removing substantially all of the water from the precursor composition. The absorbent articles are useful as surgical sponges, diapers, tampons, meat trays, bath mats, and the like.
Furthermore, U.S. Pat. No. 5,534,304 to Geursen and Willemsen, assignors to Akzo Nobel NV, shows a process for treating a substrate, namely a fibre or a fibrous product, with a superabsorbent material, in which process there is applied to the surface of the substrate, which is not an aramide fibre, a layer of a water-in-oil emulsion which contains a superabsorbent material in its aqueous phase, so that there is applied to the substrate, calculated on its dry weight, 0.3 to 40 wt. % of the superabsorbent material, after which the liquid constituents of the emulsion are wholly or partially removed from the substrate.
Also of interest are U.S. Pat. No. 4,812,491 (issued Mar. 14, 1989) to Hahn, assignor to the Glidden Company, and EP Published Patent Application No. 0 021 618 A1 (published Jan. 7, 1981) to Backhouse and Palluel, assignors to Imperial Chemical Industries Limited, both of which describe suspension polymers useful as paint coatings. More particularly, the U.S. patent involves a process for producing a self-curing paint coating composition that requires co-polymerizing ethylenically unsaturated monomers, such as carboxyl or hydroxyl functional monomers and also alkylol acrylamide monomer (but excluding amine monomers), followed by treating the resultant with an ion exchange resin to remove cations from the emulsion polymer and produce a pH of less than 2.5 in order to produce a thermoset paint coating film. The European publication involves a process for the production of X-linked addition polymer microparticles made from the dispersion polymerization of ethylenically unsaturated monomers in an aliphatic hydrocarbon liquid in the presence of a specific stabilizer. The process requires that at least one of the monomers contains hydroxymethylamino or alkoxymethylamino and at least one other monomer contains hydroxyl or carboxyl. The resultant microparticles are incorporated into paint coating compositions.
As is well known, one use for superabsorbent polymers is in making tapes for water blocking when a tape is placed alongside fiber optic filaments in an end use communications cable. Thus, additionally of note is U.S. Pat. No. 5,642,452 (issued Jun. 24, 1997) to Gravely, Stokes. and Tanaka, assignors to Sumitomo Electric Lightwave Corporation. This patent describes an optical fiber communications cable that is manufactured absent the use of a viscous water blocking compound. Rather, a water swellable yarn is helically wrapped around the central strength member in order to absorb water that may become present between the strength member and the buffer tubes of the cable. Also, a water swellable tape is disposed between the core and the jacket of the cable to absorb water that may become present between the buffer tubes and elements overlaying the core of the cable. The patent describes the strength member as being formed from aramide or glass, and most suitably formed of KEVLAR(copyright), a trademark for a type of aramide sold by DuPont Corporation of Wilmington, Del. Also, the patent describes that the water blocking yarn and water swellable tape are made from polyester.
The disclosures of all of the above-mentioned patents and published patent applications are incorporated by reference.
Nevertheless, a need still exists to obviate making tapes of superabsorbent polymer particles, as described in U.S. Pat. No. 5,642,452 mentioned above. Industry would be given a great advantage if strands of various materials (or alternatively, the fiber optic filaments) could be dip coated into an aqueous solution of the pre-superabsorbent polymer, followed by heating the polymer in order to convert it into a superabsorbent polymer coating on the strands (or alternatively, on the fiber optic filaments). In particular, coating the fiber optic filaments obviates the many problems with placing the tapes and fiber optic filaments together.
Therefore, the present invention provides a method for making a composite of a substrate having an application of superabsorbent polymer that is adhered to the substrate. The method comprises first preparing an aqueous solution of a polymer which is capable, upon being subjected to radiation from a radiation source for a sufficient time, of becoming a superabsorbent polymer. Preferably, the radiation source provides UV rays and/or heat. Then, the aqueous solution is applied to a substrate. Next, the resultant of the substrate with the applied aqueous solution is subjected to the radiation source, for instance heated for a sufficient time at a sufficient temperature, to convert the polymer to a superabsorbent polymer. Obtained is a composite of a substrate having a coating of superabsorbent polymer adhered to the substrate. Typically, the superabsorbent polymer is water insoluble.
Preferably, the polymer has functional groups that cross-link upon being subjected to the radiation source, such as UV rays and/or heat, in order to form the superabsorbent polymer. More preferably, in one embodiment, the polymer has a network cross-linking agent that is an alkylol methacrylamide, which is believed to allow for heating at relatively low temperatures of about 140xc2x0 C. or less for the conversion to a superabsorbent polymer.
Accordingly, it is an object of the present invention to provide a method for making composites of a substrate coated with a superabsorbent polymer, wherein the preferred low temperature embodiment saves energy as compared to high temperature methods.
Hence, advantageously with the lower temperature embodiment, the curing oven, that is used for converting the polymer to a superabsorbent polymer via heat, can have a higher throughput.
Furthermore, it is an advantage of the lower temperature embodiment that many more kinds of substrates that would be ruined by the higher temperatures can be used.
Another advantage of the lower temperature embodiment is that the superabsorbent polymer component of the composite often exhibits a desirably high CRC, and typically, the CRC with Dl water will be approximately 45 g/g.
One more advantage, particularly with the higher temperature embodiment, is sea water stability of the superabsorbent polymer, as further illustrated in Example C below.
Some of the objects and advantages of the invention having been stated, other objects and advantages will become evident as the description proceeds, when taken in connection with the Laboratory Examples described below.